Bistable hydraulic transfer means



July 23, 1968 s. EscoaosA 3,393,693

BISTABLE HYDRAULIC TRANSFER MEANS Filed April 1, 1964 5 Sheets-Sheet 1 BISTABLE TRANSFER MEANS SECOND PRESSURE SOURCE INVENTOR. ALFONSO S. ESCOBOSA ATTORNEY July 23, 1968 A. s. ESCOBOSA 3,393,593

BISTABLE HYDRAULIC TRANSFER MEANS Filed April 1. 1964 a Shets-Sheet 2 N VEN TOR. ALFO S ESCOmSA ATTORNEY 3 Sheets-Sheet 3 July 23, 1968 A. s. ESCOBOSA BISTABLE HYDRAULIC TRANSFER MEANS Filed April 1, 1964 INVENTOR. ALFONSO S ESCOBOSA 474% I/ I N m A 2N -(7 w m// W//// T/// 5 z num E 3 ATTORNEY United States Patent() 3,393,693 BISTABLE HYDRAULIC TRANSFER MEANS Alfonso S. Escobosa, Placentia, Califi, assignor to North American Rockwell Corporation, a corporation of Delaware Filed Apr. 1, 1964, Ser. No. 356,458 4 Claims. (Cl. 137-113) The present invention relates generally toa dual source hydraulic system, and more specifically to a bistable transfer device for alternatively allowing fluid pressure from dual pressure sources to actuate the load portion of an hydraulic system. This improved 'dual source hydraulic system is adapted to provide an uninterrupted supply of source pressure despite a pressure .failure in one ofthe dual sources.

In the dual hydraulic system of manned aircraft, dual tandem flow metering valves and dual tandem actuators are-commonly utilized. In the event of a failure causing partial or total loss of hydraulic supply pressure, the operative halves of the flow metering valves and actuators will continue to provide the proper control displacements. However, in' the act of providing the control displacements, the hydraulic fluid in the failed side of the actuator will be forced to throttle through the flow metering valve and thereby reduce the response performance of the servo. This reduced response may be overcome by the use of bypass valves which provide a bypass for the hydraulic fluid. This represents a distinct disadvantage, however, due to'the added complexity introduced by the bypass valves. The present invention eliminates the need for complex bypass valves and will replace the dual metering valves and dual actuators with a single valve and actuator,

An object of this invention is to provide means for disconnecting a'load portion of an hydraulic system from a failed source of hydraulic fluid pressure and for immediately connecting the load portion to an operating source of fluid pressure.

Another object of this invention is to provide a simple and rapid means for transferring connections from one high pressure hydraulic'source to another.

A further object of this invention is to provide bistable means for alternatively connecting one or the other of two hydraulic fluid sources to the load portion of an hydraulic system wherein the transfer is accomplished upon the decrease of one source to a predetermined level below the pressure of the other source.

These and other objects are achieved through the use of a bistable hydraulic valve means. The valve means is adapted to be connected to a pair of hydraulic sources to control the flow of actuating fluid to the load portion of the system from only one of the sources at a time. An automatic transfer from one source of fluid to another is accomplished in the event of an operational failure of the one source, thereby providing high reliability to the system. The bistable valve means therefore assures an almost constant supply of source pressure to the load since the transfer is performed in a matter of a few milliseconds. The operation of the valve means is controlled by a pilot valve which 'is slidable from one end to the other within its valve bore. Each valve and surface area is acted upon by a different source of pressure, but when in either of the two stable states the area being acted upon is not equal. An annular area B, to be found on both ends of the valve, rests against the valve bore leaving only a recessed area A exposed to one of the two source pressures. The other source pressure will be operating on the areas A+B of the opposite end of the valve. The valve remains in this position until the source acting on areas A+B falls to a level such that the force exerted on areas A-l-B is less than that exerted by the one pressure source on area A alone. This relation gives rise to a switching ratio which may be defined as In other words, when the actuating pressure becomes. less than the standby pressure times the switching ratio the valve will shift. After a small increment of movement the newly actuating pressure of theone source is able to act upon area B as well as area A, thereby further increasing the shifting force. When the valve contacts the opposite end, the other pressure source is shut off from area A and can act only upon area B. Utilizing the foregoing mode of control, the communication of fluid pressure from either of two sources to the load portion of the system is determined.

Other objects, features and advantages of this invention will become apparent from the following detailed description of selected illustrative embodiments taken in conjunction With the accompanying drawings wherein:

FIG. 1 is a diagram schematically showing the bistable transfer means in relation to the dual pressure sources and the load portion of an hydraulic system;

FIG. 2 is a sectional view of a schematic representation of one embodiment of the invention;

FIG. 3 is a sectional view of a schematic representation of a preferred embodiment of the invention wherein the porting arrangement is merely indicated but not shown;

FIG. 4 is a sectional view of a schematic representation of another preferred embodiment of the invention.

Referring now to FIG. 1, there is shown a first source 10 and a second source 11 of high pressure hydraulic fluid. These pressure sources are connected to a bistable transfer means 12 by pressure lines 13 and 14, and by return lines 15 and 16. Pressure lines 13 and 14 are connected to high pressure inlets 17 and 18. Return lines 15 and 16 are connected to low pressure outlets 19 and 20. A load portion 21 of an hydraulic system is connected to the bistable transfer means 12 by a load pressure line 22 and a load return line 23. Load pressure line 22 is connected to a high pressure outlet 24 and load return line 23 is connected to a low pressure inlet 25.

In operation, high pressure fluid is transmitted from the first source 10 through the pressure line 13 to the high pressure inlet 17, and then through the bistable transfer means 12 to the high pressure outlet 24 and from there through the load pressure line 22 to the load portion 21 of the hydraulic system. The low pressure return path through the line 23 from the load portion 21 enters inlet 25 and passes through the bistable transfer means 12 to the low pressure outlet 19 and through the return line 15 to the first high pressure source 10.

Fluid communication through the bistable transfer means 12 will continue along the paths between the high pressure inlet 17 and the high pressure outlet 24, and between the low pressure inlet 25 and the low pressure outlet 19, so long as the first source 10 of high pressure fluid continues to maintain its pressure above a predetermined level relative to the pressure level of the second high pressure source 11. When functioning in this manner, the bistable transfer means 12 is in its first position.

In the event of a partial or total failure of the pressure in the first source 10, the pressure level in the line 13 decreases when that pressure falls below the pressure in the second source 11 by a predetermined amount, the bistable transfer means automatically switches to its second position connecting the high pressure inlet 18 with the high pressure outlet 24, and the low pressure inlet with the low pressure outlet 20. Fluid will now be communicated from the second source 11 through the pressure line 14 and returned through the line 16. The first source 10 may then be repaired or replaced to provide a standby source which may be automatically switched to the load 21 by the bistable transfer means 12 if the second source should fail.

The concept just described with reference to FIG. 1 has particular application in the hydraulic control system of both air-borne and spacecraft vehicles. It may be used in conjunction with hydraulic servo actuators to greatly increase their reliability. The bistable transfer means 12 will switch from one pressure source to the other only when a predetermined pressure ratio between the two supplies occur. A typical ratio might be 3/4.

Referring now to FIG. 2, a schematic representation of the bistable transfer means 12 is shown having one pair of valves 30, 31 which operate to translate hydraulic pressure between the first pressure source 10 and the load 21, not shown. As shown, valves and 31 are in their mid-positions within respective valve bores 32 and 33. When in their open position, valves 30 and 31 allow return communication between a low pressure inlet 27 and the low pressure outlet 19 and pressure communication between the inlet 17 and high pressure outlet 26. The first high pressure source is connected to high pressure inlet 17 and to low pressure outlet 19, as shown in FIG. 1 and the hydraulic load is connected to high pressure outlet 26 and to low pressure inlet 27.

Another pair of valves 34, operate to control the transfer of hydraulic pressure between the second pressure source and the load. Valves 34 and 35 are also shown in their mid-positions within respective valve bores 36 and 37. When in their open positions, valves 34 and 35 allow communication between the high pressure inlet 18 and a high pressure outlet 28 and between a low pressure inlet 29 and the low pressure outlet 20. The second high pressure source is connected to high pressure inlet 18 and to low pressure outlet 20, and the hydraulic load is connected to high pressure outlet 28 and to low pressure inlet 29.

A pilot valve mounted within a valve bore 41 is utilized to provide a means for controlling the positions of the two pairs of valves 30, 31 and 34, 35. Pilot valve 40 is also shown in its mid-position.

For purposes of explanation, when pilot valve 40 is in contact with surface 38 of valve bore 41, it will be said to be in its first position. When it is in contact with surface 39 of valve bore 41, it will be said to be in its second position. Valve bore 41 contains a high pressure inlet 42 which is connected to the first high pressure source, and a high pressure inlet 43 which is connected to the second high pressure source. The pressure entering high pressure inlet 42 tends to force pilot valve 40 to the right into its first position and the pressure entering through high pressure inlet 43 tends to force pilot valve 40 to the left into its second position.

When pilot valve 40 is in its first position, high pressure P from the first source bears upon surfaces 44 and 45. High pressure P from the second source bears only upon surface 46 while the low return pressure P to the second source acts upon surface 47 through a low pressure inlet 48 and passageway 50 within pilot valve 40. Since the first high pressure source is acting upon a larger area than the second pilot valve 40 will remain in its first position until such time as the first source pressure P falls to some set level below the pressure P in the second source. The switching point is reached When the product of the first high pressure P times the combined area (A i-B of surfaces 44 and becomes less than the product of the second high pressure P times the area of A of surface 46 plus the return pressure P 4 of the second source times the area B of surface 47. If both A and A are equal to A, and both B and B are equal to B, the switching ratio may be defined by the relation by assuming that the practical effect of the return pressure of the second source is negligible. Therefore, if the pressure in the first source decreases below the pressure in the second source, and reaches the level at which the ratio between the two pressures equal the switching ratio, the pilot valve 40 will shift to the second position.

With pilot valve 40 in the second position, the above situation is exactly reversed. Fluid from the second high pressure source acts upon the combined areas (A +B of surfaces 46 and 47 and this force is only counteracted by any remaining high pressure in the first source acting upon surface 44 plus the low return pressure of the first source acting upon surface 45. This low pressure contact is made possible by means of a low pressure inlet 52 and a passageway 53 within pilot valve 40.

Assume for the moment that pilot valve 40 is being shifted into its first position. High pressure fluid P entering inlet 42 will pass through valve bore 41 and be communicated through connecting means and 61 to valve bores 36 and 37. The high pressure fluid acting upon surfaces 62 and 63 of the respective valves 34 and 35 will force those valves into their closed position. Communication is thereby cut off between inlet 18 and outlet 28, and between inlet 29 and outlet 20. At the same time, valves 31 and 32 will be shifted into open position by high pressure fluid from the first source entering inlet 17 acting upon surface 64 of valve 31 to provide a path to outlet 26, and by return pressure entering inlet 27 acting upon surface 62 of valve 30 to provide a path to outlet 19. Thus, with the first high pressure source operating and the valve 40 in its first position, the fluid path will be from the first source connected to inlet 17 through valve bore 33 to outlet 26, and from the load connected to inlet 27 through valve bore 32 to outlet 19 which is connected to the first source by the line 15 shown in FIG. 1.

When the pressure P of the first source decreases to a predetermined level below the pressure P of the second source, pilot valve 40 will automatically switch to its second position. This switching action is accelerated by the presence of a more positive pressure from the second source since, after a slight displacement, the high pressure P from the second source is able to act upon both the area of surface 46 and the area of surface 47. At the same time, displacement of the valve 40 cuts off fluid communication between inlet 43 and return line 48 through passageway 50.

The high presure fluid entering inlet 43 passes through valve bore 41 and is communicated through connecting means 66 and 67 to valve bores 32 and 33. The high pressure P acting upon surfaces 68 and 69 will force valves 30 and 31 into their closed position, thereby cutting off communication between inlet 17 and outlet 26, and between inlet 27 and outlet 19. High pressure P from the second source at inlet 18 acts upon surface 70 to open valve 34 and provide high pressure at outlet 28. Return pressure at inlet 29 acts upon surface 71 to open valve 35 and provide a return path through outlet 20. Thus, with the second high pressure source operating, the fluid path will be from the second source to inlet 18, through valve bore 36 to outlet 28, and from there to the load, thereafter returning from the load to inlet 29, through valve bore 37, and emerging from outlet 20 to return to the point of origin through line 16 shown in FIG. 1.

Valve bore 32 is also connected to the second pressure source at inlet for two purposes, First, in order that the remaining pressure from the second source may act upon surface 81 to hasten and assure the opening of valve 30, which is also being forced open by the operating return fluid pressure entering inlet 27. Second, in order that inlet 80 may function as a means of escape for any fluid which may have leaked into the'enclosed volume behind surface 81. In like manner, valve bore 37 is connected to the first pressure source by inlet 86. The closing of valve 35 is aided by the action of any remaining first source pressure upon surface 87. Inlet 86 also provides an escape path for any fluid which may leak behind surface 87, which fluid will be forced out of valve bore 37 upon the closing of valve 35. Outlet 82 connects valve bore 33 to the second source return line and provides an escape path for any fluid which may have leaked behind surface 83. Such fluid will be forced out of valve bore 33 upon the closing of valve bore 31. Outlet 84, connected to the first source return line, functions in like manner, allowing fluid trapped behind surface 85 to escape from valve bore 36 upon the closing of valve 34.

Though not shown in FIG. 2, high pressure outlets 2-6 and Y28 are subsequently interconnected before being coupled to the load. This connection is represented by outlet 24 in FIGURE 1. Similarly, return pressure inlets 27 and 29 stem from a common connection represented by inlet 25 in FIGURE 1.

Referring now to FIGURE 3, a schematic drawing is shown representing a preferred embodiment of the invention. The actual porting arrangements are not shown, but merely indicated. The bistable transfer means 12 is shown with its valves 103, 113 and 123 at their mid-positions within their valve bores 100, 110 and 120. Fluid from the first high pressure source, passing through line 13, enters valve bore. 100 between lands 101 and 102 on valve 103. Fluid from the second high pressure source, passing through line 14 enters valve bore 100 between lands 102 and 104. Low pressure fluid returning to the first source through line is exhausted from the valve bore 110 between lands 111 and 112 on valve 113. Low pressure fluid returning to the second source through line 16 is exhausted from the valve bore 110 between lands 112 and 114.

The valve bore 120 is connected at opposite ends to line 42 carrying first high pressure fluid and to line 43 carrying second high pressure =fluid. The pilot valve 123 is mounted within the valve bore 120. The first low pressure return communicates through line 124 with the valve bore 120, and the second low pressure return communicates through line 125 with the valve bore 120'. Valves 103, 113 and 123 are each slidable between first and second positions within valve bores 100, 110 and 120, respectively. The valve 103 is in its first position when surface 130 is in contact with surface 131. It is in its second position when surface 132 is in contact with surface 133. The valve 113 is in its first position when surface 140 is in contact with surface 141, and it is in its second position when surface 142 is in contact with surface 143, The valve 123 is in its first position when surface 150 is in contact with surface 151. It is in its second position when surface 152 is in contact with surface 153.

Assume that the bistable transfer means 12 is in operation and transferring high pressure fluid from the first source to the load. Each of the valves 103, 113 and 123 will be in their first position. High pressure fluid entering the valve bore 100 from line 13 will be communicated to the load through line 22 since this path is now no longer blocked by land 102 as shown when the valve 103 is in its mid-position. With the valve 113 in its first position, return fluid from the load enters the valve bore 110 from line 23 between lands 111 and 112 and is exhausted through line 15 to return to the first source. This return path is blocked by land 112 when the valve 113 is in its mid-position.

With the valve bore 123 in its first position, first high pressure fluid through line 42 enters the valve bore 120 and acts upon surfaces 155 and 156 of the valve 123 to maintain it in its first position. High pressure fluid also passes from the valve bore 120 into valve bores and through communication means 160 and 161 and acts upon surfaces 132 and 142 to maintain respectively valves 103 and 113 in their first positions. Fluid from the second high pressure source will act upon surface 165 of the valve 123 and tend to force the valve toward its second position. However, so long as the first source pressure is maintained above a predetermined level, the valVe'123 cannot shift, since the area of surface 165 is smaller than the area of surfaces 155 and 156.

In the event of a partial or total failure of the first pressure source, the bistable transfer means 12 will switch in order to provide a communication path between the second source of pressure and the load. This switching willoccur when the force exerted by the second high pressure fluid, entering into the valve bore through line 43 acting upon surface 165 exceeds the-force exerted by the first high pressure fluid acting upon the combined area of surfaces 155 and 156, As the valve 123 begins to shift towards its second postion, low pressure communication is opened up between line 124 and surfaces 155 and 156 through passageway 167. This further reduces the force opposing the shifting of the valve 123. Even before the valve 123 reaches its second position, fluid paths are opened through communication means 170 and 171 to allow second source high presure fluid to enter valve bores 100 and 110. The second source high pressure fluid then acts upon surfaces and to shift respectively valves 103 and 113 to their second positions.

When in its second position, valve 103 provides a communication path between the second high pressure source and the load. The second source high pressure fluid enters valve bore 100 through line 14 between lands 102 and 104 and exits valve bore 100 through line 22. When in its second position, the valve 113 provides a communication path for the return fluid from the load to the second source of pressure. The return fluid enters the valve bore 110 through line 23 between lands 112 and 114 and is exhausted through line 16 to return to the second source.

The bistable transfer means 12 will remain in its second position so long as the second source pressure is maintained above a predetermined level somewhat below the pressure remaining in the first source. If the second pressure should fall below this predetermined level, a switching operation similar to that just described will occur, but in the opposite direction. In other words, when the force of the first pressure acting upon the area of surface is greater than the force exerted by the second pressure source upon the combined area of surfaces and 166, the pilot valve 123 will shift to its first position. Positive switching pressure is realized as before when low pressure communication is opened to surfaces 165 and 166 through passageway 168 thereby decreasing the force which is resisting the motion of valve 123.

Referring finally to FIGURE 4, another preferred embodiment of the invention is schematically shown. In this embodiment all of the features of the present invention are incorporated within the framework of one valve bore 200 and one valve 201. High pressure fluid from the first pressure source enters bistable transfer means 12 at inlet 17. High pressure fluid from the second source enters at inlet 18. High pressure fluid emerges from transfer means 12 through outlets 26 and 28, alternatively, and combines at junction 24 then to pass through line 22 to the load. Fluid returning from the load through line 23 enters transfer means 12 at inlet 25. Low pressure fluid is exhausted,

alternatively, through outlet 19- and line 15 to the first source, and low pressure fluid through outlet 20 line 16 to the second source. The valve 201 is shown in its first position within the valve bore 200'. In this position, the first source of fluid pressure is operating. High pressure fluid enters inlet 17 and passes through the valve bore 200 to outlet 26 and then to the load. A low pressure return path, from inlet 25 through the valve bore 200' to outlet 19, is also provided when the valve 201 is in its first position. The high pressure outlet 28 is closed off by thedand 205. By means of passageways 210 and 211, first high pressure source fluid is able to act upon surface 212. Force exerted against areas 212 and 213 by the first source pressure will maintain the valve 201 in its first position. If, however, the pressure in the first source fails or decreases to a predetermined level below the pressure in the second source,the valve 201 will then be shifted to a second position at the opposite end of the valve bore 200. The shift occurs when the force exerted by the second pressure on the area of surface 215 exceeds the force exerted by the first pressure on the area of surfaces 212 and 213. As the valve 201 shifts a finite increment towards its second position, a communication path comprised of passageways 220 and 221 will begin to open to permit second high pressure fluid to act upon surface 222 thereby increasing the force which is causing the valve 201 to shift. In addition, a low pressure communication path will be opened to surface 212 through passageway 210 to outlet 19 which further decreases the force opposing the motion of valve-201. The duration of the switching mode for this configuration is on the order of milliseconds.

When the valve 201 is in its second position, a communication path between high pressure inlet 18 and high pressure outlet 28 is established. The second high pres sure fluid can now pass through the valve bore 200 and on to the load as indicated above. Low pressure return fluid from the load will re-enter at inlet 25 and pass through the valve bore 200 to be exhausted through outlet 20 to return to the second source. The high pressure outlet 26 is closed off by the land 206. The valve 201 will remain in its second position so long as the second source of pressure is operative. Should the pressure in the second source decrease to the prescribed level below the remaining pressure in the first source, valve 201 would shift back to its first position. The operation would merely be the reverse of the switching operation described above.

' While the principles of the invention have now been made clear in the illustrative embodiments, there will be immediately obvious to those skilled in the art many modifications in structure, arrangement, proportions, the elements, materials, and components, used in the practice of the invention, and otherwise, which are particularly adapted for specific environments and operating requirements, without departing from those principles. The appended claims are therefore intended to cover and embrace any such modifications, within the limits only of the true spirit and scope of the invention.

What I claim is:

1. The combination comprising a housing having a valve bore therein,

a valve shiftably mounted between first and second positions in said bore,

a plurality of high pressure inlets and low pressure outlets adapted to be connected to said first and second high pressure hydraulic sources,

first and second high pressure outlets and a low pressure inlet adapted to be connected to a load portion of an hydraulic system,

a plurality of passageways within and lands upon said valve, said valve more and said valve providing high pressure communication between a first pressurized source of hydraulic fluid, and said first high pressure outlet when said valve is in said first position and providing high pressure communication between a second pressurized source of hydraulic fluid and-said second high pressure outlet when said valve is in said second position, said valve bore and said valve providing low pressure communication between said first source of hydraulic fluid and said low pressure inlet when said valve is in said first position and providing low pressure communication between said second source of hydraulic fluid and said low pressure inlet when said valve is in said second position, and

control means comprises:

control means for allowing said valve to shift from one of said first and second positions to the other of said first and second positions only when the pressure in one of said first and second sources decreases to a prescribed fraction of the pressure in the other of said first and second sources.

2. The combination defined in claim 1, wherein said first and secondends of said valve, said passageways within and lands upon said valve, and said first and second high pressure hydraulic sources, said ,first high pressure source operating upon said first end and a first of said lands when .said' valve is in said first position and operating'only upon said first land when said valve is in said second position, said second high pressure source operating upon said second end and a second of said lands when said valve in in said second position and operating only .upon said second land when said valve is in said first position, said valve shifting from said first position to said second position when the force exerted by said first high pressure source acting upon said first end and said first land becomes less than the force exerted by the second high pressure source acting upon said second land, said valve shifting from said second position to said first position when the force exerted by said second high pressure source acting upon said second end and said second land becomes less than the force exerted by said first high pressure source acting upon said first land.

3. The combination comprising a valve mounted within a valve bore in said housing,

a plurality of high pressure inlets and low pressure outlets adapted to be connected to said first and second high pressure hydraulic sources,

first and second high pressure outlets and a low pres sure inlet adapted to be connected to said load portion of an hydraulic system,

a plurality of passageways within and lands upon said valve, said valve bore and said valve providing high pressure communication between said first source of hydraulic fluid and said first high pressure outlet when said valve is in said first position and providing high pressure communication between said second source of hydraulic fluid and said second high pressure outlet when said valve is in said second position, said valve bore and said valve providing low pressure communication between said first source of hydraulic fluid and said low pressure inl'et when said valve is in said first position and providing low pressure communiuation between said second source of hydraulic fluid and said low pressure inlet when said valve is in said second position, and

control means for allowing said valve to shift from one of said first and second positions to the other of said first and second positions only when the pressure in one of said first and second sources decreases to a prescribed fraction of the pressure in the other of said first and second sources. 1

4. The combination defined in claim 3, wherein said control means comprises:

first and second ends of said valve, said passageways within and lands upon said valve, and said first and second high pressure hydraulic sources, said first high pressure source operating upon said first end and a first of said lands when said valve is in said first position and operating only upon said first land when said valve is in said second position, said second high pressure source operating upon said second end and a second of said lands when said valve is'in said second position and operating only upon said second'land when said valve is in said first position, said valve shifting fronrsaid first position to said second position when the force exerted by said first high pressure source actingupon saidjlfirst end and said first land becomes less than the force exerted by the second high pressure source acting upon said second land, said valve shifting from said second position to said first position when the force exerted by said second high pressure source acting upon said second end and said second land becomes less than the force exerted by said first high pressure source acting upon said first land.

10 References Cited UNITED STATES PATENTS 8/1957 Harp 137 113 12/1965 Alfieri 137 113 

1. THE COMBINATION COMPRISING A HOUSING HAVING A VALVE BORE THEREIN, A VALVE SHIFTABLY MOUNTED BETWEEN FIRST AND SECOND POSITIONS IN SAID BORE, A PLURALITY OF HIGH PRESSURE INLETS AND LOW PRESSURE OUTLETS ADAPTED TO BE CONNECTED TO SAID FIRST AND SECOND HIGH PRESSURE HYDRAULIC SOURCES, FIRST AND SECOND HIGH PRESSURE OUTLETS AND A LOW PRESSURE INLET ADAPTED TO BE CONNECTED TO A LOAD PORTION OF AN HYDRAULIC SYSTEM, A PLURLAITY OF PASSAGEWAYS WITHIN AND LANDS UPON SAID VALVE, SAID VALVE MORE AND SAID VALVE PROVIDING HIGH PRESSURE COMMUNICATION BETWEEN A FIRST PRESSURIZED SOURCE OF HYDRAULIC FLUID, AND SAID FIRST HIGH PRESSURE OUTLET WHEN SAID VALVE IS IN SAID FIRST POSITION AND PROVIDING HIGH PRESSURE COMMUNICATION BETWEEN A SECOND PRESSURIZED SOURCE OF HYDRAULIC FLUID AND SAID SECOND HIGH PRESSURE OUTLET WHEN SAID VALVE IS IN SAID SECOND POSITION, SAID VALVE BORE AND SAID VALVE PROVIDING LOW PRESSURE COMMUNICATION BETWEEN SAID INLET SOURCE OF HYDRAULIC FLUID AND SAID LOW PRESSURE INLET WHEN SAID VALVE IS IN SAID FIRST POSITION AND PROVIDING LOW PRESSURE COMMUNICATION BETWEEN SAID SECOND SOURCE OF HYDRAULIC FLUID AND SAID LOW PRESSURE INLET WHEN SAID VALVE IS IN SAID FIRST POSITION AND PROVIDING LOW PRESSURE COMMUNICATION BETWEEN SAID SECOND SOURCE OF HYDRAULIC FLUID AND SAID LOW PRESSURE INLET WHEN SAID VALVE IS IN SAID SECOND POSITION, AND CONTROL MEANS FOR ALLOWING SAID VALVE TO SHIFT FROM ONE OF SID FIRST AND SECOND POSITIONS TO THE OTHER OF SAID FIRST AND SECOND POSITIONS ONLY WHEN THE PRESSURE IN ONE OF SAID FIRST AND SECOND SOURCES DECREASES TO A PRESCRIBED FRACTION OF THE PRESSURE IN THE OTHER OF SAID FIRST AND SECOND SOURCES. 